U.S. Pat. No. 4,144,411 issued to Frenkiel on Mar. 13, 1979, teaches static reuse of frequencies in a large-cell reuse pattern to a miniature-sized overlaid, but same type reuse pattern. Therefore, the miniature-sized reuse pattern and the large-cell patterns are both on seven-cell repeat patterns. This is achieved through yet lower transmit powers and maintaining the same site spacing to cell radius as the large-cell. This concept is referred to as cell splitting and is one method of improving frequency reuse of traffic channels in a geographic region. The decision to handoff from an overlaid serving cell to an underlaid cell (not from one reuse pattern to another at the same site) is based on whether a subscriber's received signal strength (RSS) is greater than a threshold set for the overlaid cell. If the RSS is equal to or less than the predetermined threshold, a check is made to see if there is a large-cell channel available.
An enhancement to Frenkiel is discussed in an article authored by Samuel W. Halpern entitled Reuse Partitioning in Cellular Systems, presented at the 33rd IEEE Vehicular Technology Conference on May 25-27, 1983 in Toronto, Ontario, Canada. The Halpern article sets forth a cellular system having multiple resuse levels (or patterns) within a given geographical area. A reuse level will refer to a particular channel reuse pattern whether the channel is based on frequency, time slots, codes, or other suitable divisions. For example, a cluster of cells normally employing a seven-cell reuse pattern may simultaneously operate on a three-cell reuse pattern whereby one set of frequencies is dedicated to the three-cell reuse pattern while another set of frequencies is dedicated to the nine-cell reuse pattern. This division of frequency spectrum into two groups of mutually exclusive channels is one method of providing multiple reuse levels. Consequently, one cell site may operate on both a nine-cell and a three-cell reuse pattern by using channels from its channel set which are dedicated to specific cell sites and assigned to the different reuse patterns. Such smaller reuse patterns form a noncontiguous overlay of cells having a decreased radius. Although this article discusses channels in terms of frequencies, it is well understood that channels include data channels and traffic channels, which may be time slots within the same frequency such as in a Time Division Multiple Access (TDMA) system, or traffic channels and data channels in other types of channelized systems such as Code Division Multiple Access (CDMA) systems.
Generally, the principle behind the Halpern system is to allow a degradation of C/I performance for those subscriber units that already have more than adequate C/I protection while providing greater C/I protection to those subscribers that require it. Therefore, a subscriber with the best received signal quality will be assigned to the set of channels for the three-cell reuse pattern since they are able to tolerate more co-channel interference than a subscriber whose signal quality is poorer. The subscriber having the poorer received signal quality is therefore assigned to a channel correspondent to the nine-cell reuse pattern. Halpern disclosed a ratio metric determination of how to assign subscriber units to achieve approximately a 30% increase in capacity over a seven-cell reuse level system. This resulted by maintaining 60% of the subscribers in the three-cell level while maintaining 40% in the nine-cell level. However, this reuse partitioning system as disclosed generally indicates that the received signal quality measurement is taken only by the serving cell. The serving cell then determines which reuse channel to assign the subscriber based upon the 60/40 predetermined ratio. Such a method fails to adequately account for interference from surrounding site interference whether the sites are neighboring sites, adjacent sites, or even distant sites causing interference through multipath interference or by some other impairing phenomenon. Such a system typically allocates a subscriber on its C/I relation to all other subscribers in that the 60% of the subscribers having the best C/I are assigned to the three-cell reuse pattern even though they may perform better on a different reuse level.
Other frequency reuse systems take into account all base sites within a given geographic coverage area, and then determine those sites that are allowed to transmit at the reuse frequency based upon the combined signal strength of all the measured sites. Such a system is disclosed in U.S. Pat. No. 4,670,906 by Thro, issued on June 2, 1987, and assigned to instant assignee. In large cellular systems having many base sites within a small area, such systems become computationally intensive resulting in increased processing demands and increased system complexity, particularly as more base sites are added.
In addition, systems employing dynamic multiple levels, such as that described by Schaeffer in instant assignee's U.S. application Ser. No. 07/485,718 filed Feb. 27, 1990, cause further interference complications since neighboring sites and further distant sites may cause rapidly changing interference through the continuously changing reuse patterns. A static system (as generally disclosed by Halpern) typically dedicates a set of channels to the cells of a specific reuse pattern and are not generally reassigned to a different reuse pattern in neighboring cells. Systems employing multiple dynamic channel reuse levels generally allow reuse channels to be dynamically assigned multiple times during the same conversation to neighboring cells or different reuse levels in accordance with system reuse guidelines. Therefore, the appropriate reuse level to assign the serving station or to assign the subscriber unit must be rapidly evaluated and adequately determined while still accommodating increases in system capacity.
There exists a need for a reliable and relatively rapid method for assigning channel reuse levels to subscriber units in either a static multi-level reuse system or a dynamic multi-level reuse system. Such a method should include sufficient interference measurement both uplink and downlink to provide a more reliable C/I determination by which reuse levels are assigned to appropriate serving sites and subscribers are assigned to appropriate reuse levels.